Many types of electronic test equipment (e.g., oscilloscopes) often involve the probing of a circuit of interest with a hand held probe. The probe might acquire a single-ended signal or a differential one, and there may or may not be a ground connection using a “flying lead” (a short length of flexible insulated wire with an alligator clip or other fastener at the free end). In particular, it is often necessary to probe the signals at two places on a PCB (Printed Circuit Board) that: (1) Are some arbitrary distance apart; (2) Are traces leading to surface mounted components with no leads around which a probe tip may be hooked, requiring that sharp probe tips be pressed into those traces; and (3) Carry signal that have high frequency components (say, in the Giga Hertz region).
To accomplish these tasks a number of desirable properties of such a probe have been identified, and various designs have been offered. These desirable properties include adjustable spacing between a pair of small sharp probe tips with spring loading. They are small to cooperate with high frequency operation. They are sharp to allow them to penetrate any protective coatings and stay in place by slightly gouging into the trace. At least one is spring loaded to help them stay in place and not slip, even though the operator's hand may move or wiggle slightly during the measurement.
A prior art micro-browser meeting these requirements has pair of rods each entering a corresponding bore in a sleeve that retains them, and that may itself be carried by a grip shaped to be rotated between a thumb and a finger. The edge of a small circuit board is soldered at the distal end of the rod. One of the rods is allowed to rotate within its bore in the sleeve, while the other is held stationary by a notch in the sleeve. The rotatable rod has a captive spring that resists the force of probe contact. Each board carries a coupling network connected to a short sharp probe tip bent downward and away from the plane of its board. The probe tips are offset from the axes of the rods, allowing the distance between the probe tips to be a function of the amount of rotation. The other end of each coupling network is coupled to a short length of a respective 50 Ω coaxial cable that passes through an axial slot in the grip to enter an amplifier pod that drives a main cable leading to test equipment. The rods may be held within the bores by friction created by slight bends in the rods. The circuit boards each include shields connected together at a location that is as close as possible to the probe tips by arranging that the rods touch each other near the probe tips. If the rods are parallel, then there is a slight bend in the non-rotating rod at the location where it passes the non-probe-carrying edge of the circuit board, such that its tip touches the tip of the rotatable rod. If the rods are both straight, then the axes of the rods are coplanar but convergent proximate the probe tips.
In operation, the spring loaded rotatable probe tip is pressed against an intended location. Once contact is made with the rotatable probe tip, further rotation of the grip also rotates the sleeve, which in turn causes an eccentric rotation of the stationary probe tip that varies the spacing between the two probe tips (the two probe tips are not along extensions of the axes of the rods). By moving the grip in a circular path (orbiting) without rotation the general orientation of the stationary probe tip relative to the other can be controlled. When both the correct spacing and the correct general orientation are achieved by a combination of orbiting and rotation, the stationary probe tip will then be positioned above the other location to be probed. A “tilting” of the entire micro-browser without further rotation or orbiting will lower the stationary probe tip onto the target location.
It is anticipated that the prior art micro-browser mentioned in the preceding paragraphs will be usable up to 10 or 12 GHz. Accordingly, it is small; the circuit boards are about 0.110″ wide and only 0.400″ long. The rods to which these boards are mounted are on the order of 1/32″ in diameter. Its usage model departs considerably from what many operators are used to, and while it does not take long to get used to the manner in which rotation and orbiting are used to achieve probe tip contact, it can take a while for some persons to appreciate that the micro-browser is, well, delicate. Not every user is a clumsy gorilla, but it is hard to make small things strong. In short, bad things happen when the user accidently pushes too hard on the micro-browser.
We have seen bent probes in similar browsers that withstand several pounds of force. Forces in the range of five to six ounces can damage the micro-browser described above.
Failures resulting from excessive contact force include bent rods, broken solder joints that attach the rod to the circuit board, dislodged probe tips and fractured circuit boards. Replacement and repair of micro-browsers that have been damaged through the accidental application of excessive force is a major aggravation for both the manufacturer and his customer. The customer is without the business end of his expensive active probe, while the manufacturer is hesitant to charge the actual cost of replacement (the micro-browser itself has only passive components, is not truly “precision” in the ususal sense of the term, and appears to the user to be mostly a mechanical interface). We need to “gorilla-proof” the micro-browser. What to do?